When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Such pacers coordinate atrial and ventricular contractions to improve pumping efficiency. Cardiac rhythm management systems also include coordination devices for coordinating the contractions of both the right and left sides of the heart for improved pumping efficiency.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by some patients is hypotension, that is, low blood pressure. Hypotension can result in dizziness, sometimes referred to as presyncope. Hypotension can even lead to unconsciousness, sometimes referred to as syncope. One cause of hypotension is an excess shifting of blood in the circulatory system toward the extremities (arms and legs) and away from vital organs in the patient's head and thorax. This can occur, for example, when the patient changes posture from lying horizontal or sitting with legs elevated to a position in which the patient is sitting or standing erect. Hypotension resulting from such changes in posture is referred to herein as orthostatic hypotension. However, hypotension may also have causes other than changes in posture. For example, maintaining the same posture for an extended period of time (e.g., sitting erect during an intercontinental airplane flight) may also cause hypotension. Moreover, certain cardiovascular disorders may result in hypotension independent of postural changes, or may exacerbate orthostatic hypotension.
For example, disautonomic syncope is a problem with the autonomic nervous system. In normal patients, the autonomic nervous system constricts the blood vessels in the extremities in response to a change to a more upright posture. This venoconstriction of the blood vessels in the extremities reduces the amount of blood that would otherwise shift to the extremities when the patient changes to a more upright posture. In some patients, however, this response by the autonomic nervous system is absent, or is even reversed by a venodilation of blood vessels in the extremities. Such patients are likely to experience hypotension. Moreover, this deficient response by the autonomic nervous system may occur even without changes in posture, leading to hypotension that is not necessarily orthostatic in nature.
Another example of a cardiovascular cause of hypotension is vasovagal syncope. In normal patients, a change to a more upright posture results in an increased heart rate. For example, for a patient that is at rest, the heart rate may temporarily increase from 60 beats per minute (bpm) to 80 bpm when the patient stands up after laying horizontally. In some patients, however, this autonomic response is absent-resulting in a drop in heart rate. This may also lead to hypotension as blood shifts away from the head and thorax into the extremities. Regardless of the cause of hypotension, the resulting symptoms of dizziness or loss of consciousness may be extremely dangerous. This is particularly so for elderly patients who are at increased risk of injury from a fall resulting from the dizziness or loss of consciousness. Hypotension is also an obvious danger for persons operating motor vehicles or other machinery. For these and other reasons, there is a need to treat hypotension to avoid these symptoms and associated risks.